In the following description and in the subsequent claims, the terms: cross-linkable unsaturated chain polymeric base, are used to indicate any non cross-linked polymer, either natural or synthetic, capable of assuming all the physico-chemical and mechanical characteristics typical of elastomers as a result of cross-linking (vulcanization).
As is known, in the manufacture of vehicle tires and, more particularly, of tires having very high performances, one of the more difficult problems to solve has always been that of achieving either a low rolling resistance at the temperatures of normal use of the tires (40-70.degree. C.), or an adequate road holding (grip) when the tires exceeds said temperatures as a result of high thermo-mechanical stresses, such as, for instance, those ensuing from a so-called "borderline" use of said tires.
It is also known that the difficulty of simultaneously achieving these desired characteristics essentially stems from the fact that they are affected in a completely opposite manner by the hysteretic characteristics, i.e. by the amount of dissipated energy, of the rubber composition that constitutes the tires tread.
In order to achieve the desired low rolling resistance of the tires, in fact, it is necessary to use rubber compositions with a low hysteresis value, and as such suitable to dissipate limited energy amounts during rolling.
On the other hand, in order to achieve the desired high road grip, it is necessary to use rubber compositions having a high hysteresis value, and as such suitable to dissipate energy amounts apt to ensure a high grip between the tread and the ground.
The solution to this problem is even more difficult if it is considered that the rubber composition constituting the tires tread should possess hysteretic behaviours quite opposite and incompatible with one another, depending upon the working temperature range of the tires.
All the attempts made in the art to improve tires performances within a temperature range above those normally used, in fact, have met either a marked and undesired increase in rolling resistance, or a substantial worsening of the abrasion resistance characteristics of the tread.
These disappointing attempts were substantially based on the use in the rubber composition of hydrocarbon resins substantially soluble in the SBR-containing polymeric bases typically used in the production of treads for vehicle tires, such as for example alpha-methylstirene-derived resins (such as the resin marketed under the trade name KRISTALEX.TM. F-85, produced by Hercules, Wilmington, Del., U.S.A.) or coumarone-based resins.
The ensuing practical consequence is that the above problem has not been solved yet in the art.